1. Field of the Disclosure
The present disclosure relates to a liquid crystal display device (hereinafter, abbreviated as LCD), and more particularly, to an array substrate for a liquid crystal display device having a light shielding film structure formed above and below a thin film transistor to block light, such as sunlight, from a high-luminance backlight, and a method for fabricating the same.
2. Background
In general, the driving principle of a liquid crystal display (LCD) device uses an optical anisotropy and polarization properties of liquid crystal. Liquid crystals have a thin, long structure, so they have orientation in an alignment of molecules, and the direction of the alignment of molecules can be controlled by intentionally applying an electric field to the liquid crystal.
Thus, when the direction of the alignment of molecules of the liquid crystal is adjusted, the alignment of molecules of the liquid crystal can be changed, and light is refracted in the direction of the molecular alignment of the liquid crystal by optical anisotropy, thus displaying image information.
Currently, an active matrix liquid crystal display (AM-LCD) (which will be referred to as an ‘LCD’, hereinafter) in which thin film transistors and pixel electrodes connected to the thin film transistors are arranged in a matrix form has come to prominence because of its excellent resolution and video implementation capabilities.
The LCD includes a color filter substrate (i.e., an upper substrate) on which a common electrode is formed), an array substrate (i.e., a lower substrate) on which pixel electrodes are formed, and liquid crystal filled between the upper substrate and the lower substrate. In the LCD, the common electrode and the pixel electrodes drive liquid crystal by an electric field applied vertically, having excellent characteristics of transmittance, aperture ratio, and the like.
However, the driving of liquid crystal by the electric field applied vertically is disadvantageous in that viewing angle characteristics are not good. Thus, in order to overcome the shortcomings, a method for driving liquid crystal by in-plane field has been newly proposed. The method for driving liquid crystal by in-plane field has excellent viewing angle characteristics.
Although not shown, the in-plane switching mode LCD is configured such that a color filter substrate and a thin film transistor substrate face each other, and a liquid crystal is interposed therebetween.
A thin film transistor, a common electrode, and a pixel electrode are formed on each of a plurality of pixels defined on the thin film transistor substrate. Also, the common electrode and the pixel electrode are separated to be parallel on the same substrate.
The color filter substrate includes gate lines and data lines formed on the thin film transistor substrate, a black matrix formed at portions corresponding to the crossings of the gate lines and the data lines, and color filters provided to corresponds to the pixels. The liquid crystal layer is driven by an in-plane field of the common electrode and the pixel electrode.
In the in-plane switching mode LCD configured as described above, the common electrode and the pixel electrode are formed as transparent electrodes in order to secure luminance, but only portions of both ends of the common electrode and the pixel electrode contribute to improvement of the luminance due to the distance between the common electrode and the pixel electrode in terms of design and most regions block light.
Thus, a fringe field switching (FFS) technique has been proposed to maximize the luminance improvement effect. The FFS technique precisely controls liquid crystal to eliminate a color shift and obtain high contract ratio, implementing high screen quality compared with the general in-plane switching technique.
The related art FFS mode LCD device having the merit of implementing such high screen quality will now be described with reference to FIGS. 1 and 2.
FIG. 1 is a plan view of a thin film transistor arrary substrate of the related art FFS mode LCD device.
FIG. 2 is a cross-sectional view taken along line II-II of FIG. 1, schematically illustrating the thin film transistor array substrate of the related art FFS mode LCD device.
As shown in FIGS. 1 and 2, the thin film transistor array substrate for the related art LCD device includes: a plurality of gate lines 13 extending in one direction and separated to be parallel on a transparent insulating substrate 11 and a gate electrode 13a extending from the gate lines 13; a gate insulating film 15 formed over the entire surface of the substrate including the gate electrode 13a; a plurality of data lines 21 formed on top of the gate insulating film 15 and defining pixel regions at the crossings of the gate lines 13 and the data lines 21; and a thin film transistor T provided at the crossing of the gate line 13 and the data line 21 and including the gate electrode 13a, an active layer 19 located above the gate insulating film 15, an ohmic contact layer 20, and a source electrode 21a and a drain electrode 21b separate from each other.
The gate electrode 13a is formed to cover the regions of the source electrode 21a, drain electrode 21b, and active layer 19 of the thin film transistor T, where a channel is formed.
Thus, the gate electrode 13a prevents leakage current generated in response to light 30 coming from below the substrate and vertically incident from a backlight.
Moreover, a pixel electrode 17 having a large area is disposed on the insulating substrate 11 in the pixel regions defined at the crossings of the gate lines 13 and the data lines 21, and a plurality of divided common electrodes 25 are disposed on top of the pixel electrode 17 to be separated from each other, with a passivation film 23 interposed therebetween.
The pixel electrode 17 overlaps the divided common electrodes 25, and is directly connected to the drain electrode.
According to the thus-configured thin film transistor array substrate for the related art LCD device, when a data signal is supplied to the pixel electrode 17 via the thin film transistor T, the common electrodes 25 to which a common voltage is supplied form a fringe field, making the liquid crystal molecules arranged in a horizontal direction between the substrate 11 and the color filter substrate (not shown) rotate according to dielectric anisotropy. The transmittance of light that transmits through the pixel regions vary according to the degree of rotation of the liquid crystal molecules, thus implementing gray scales.
According to the thus-configured array substrate for the related art FFS mode LCD device, the gate electrode is formed to cover the source electrode, drain electrode, and active layer regions of the thin film transistor T, where a channel is formed. As such, as shown in FIG. 2, the gate electrode prevents leakage current generated in response to light 30 coming from below the substrate and vertically incident from the backlight.
In the thin film transistor for the related art LCD device, however, the source electrode, drain electrode, and active layer regions where the channel is formed are directly exposed by light 40, such as sunlight, coming from an outside environment, whereby light scattered or reflected inside the device, including the light incident from the outside, enters the channel.
Accordingly, according to the related art, it is not possible to prevent the light 40 scattered or reflected inside the device, including the light incident from the outside, from being incident from above the thin film transistor and entering the channel. As a result, leakage current is generated, and this makes it impossible to display a proper image under the driving condition of the display device. Especially, when the channel portion of the active layer is exposed to light, it fails to function as a channel because of the generation of leakage current.
Consequently, it is not possible to control various voltages required for the driving of the LCD device, thereby lowering display performance.